Micromechanical devices are known having a micromirror structure manufactured using the semiconductor technology.
These micromechanical devices are used in portable apparatuses, such as, for example, portable computers, laptops, notebooks (including ultra thin notebooks), PDAs, tablets, and smartphones, for optical applications, in particular for directing light radiation beams generated by a light source with desired modalities.
By virtue of their small dimensions, these devices can meet stringent requirements as regards bulk, both as area and thickness.
For instance, micromechanical mirror devices are used in miniaturized projector modules (so-called picoprojectors), which are able to project images at a distance or to generate desired light patterns.
In combination with an image capture module, a projector module of this kind, for example, forms a three-dimensional (3D) photo- or videocamera for three-dimensional imaging. Alternatively, projector modules may be used in 3D-scene reconstruction systems that measure the time taken by a monochromatic ray emitted by the picoprojector to strike a surface and to be reflected backwards, towards a receiver (so-called time-of-flight method). Another application measures the position of the reflected ray or beam, for example of an infrared type, on an array of detectors, where the position of the reflected ray or beam depends upon the distance of the reflecting surface (so-called structured light deformation method).
Micromirror devices generally include a mirror element suspended over a cavity and formed from a semiconductor body so as to be movable, for example, with a roll and pitch movement, to direct the incident light beam as desired.
For instance, FIG. 1 schematically shows a picoprojector 9 comprising a light source 1, such as a laser source, generating a light beam 2 of three monochromatic beams, one for each base color, which, through an optical system 3 shown only schematically, is deflected by a mirror element 5 towards a screen 6. The mirror element 5 is of a two-dimensional type, controlled so as to turn about a vertical axis A and a horizontal axis B. Rotation of the mirror element 5 about the vertical axis A generates, on the screen 6, a fast horizontal scan, as illustrated in FIG. 2. Rotation of the mirror element 5 about the horizontal axis B, perpendicular to the vertical axis A, generates a slow vertical scan, generating as a whole a sawtooth scan.
In the scene reconstruction system of FIG. 3, instead, a source 11, for example a monochromatic infrared source, generates a light ray 12, which, through an optical focusing system 13, shown only schematically, is deflected by the mirror element 5 towards an object 14 and reflected by the latter onto a detector 15. A controller 16, connected to the source 11, to the mirror element 5 and to the detector 15, determines the time of flight used for scene reconstruction. Alternatively, as mentioned, the scene may be reconstructed via the structured light deformation method.
Rotation of the mirror element 5 is controlled via an actuation system, which, currently, is of an electrostatic, electromagnetic, or piezoelectric type.
For instance, FIG. 4 shows a mirror element 5 with electromagnetic actuation. Here, a chip 20 comprises a platform 21, suspended over a substrate (not visible), which has a reflecting surface (not illustrated) and is supported by a suspended frame 23 by a first pair of arms 22 (first torsion springs). The first arms 22 extend from opposite sides of the platform 21 and define rotation axis A of the mirror element 5. The suspended frame 23 is connected to a fixed peripheral portion 25 of chip 20 via a second pair of arms 26 (second torsion springs), which enable rotation of the suspended frame 23 and of the platform 21 about the horizontal axis B. The first and second arms 22, 26 are coupled to respective actuation assemblies 28A, 28B of an electrostatic type. Each actuation assembly 28A, 28B here comprises first electrodes 29 facing respective second electrodes 30.
In detail, the first electrodes 29 are fixed with respect to the respective arms 22, 26 and are comb-fingered with respect to the second electrodes 30 so as to generate a capacitive coupling. Due to the arrangement of the electrodes 29, 30 of each actuation assembly 28A, 28B, the driving structure is also defined as comb-drive structure.
By applying appropriate voltages between the first electrodes 29 and the second electrodes 30, it is possible to generate attraction/repulsion forces between them and thus cause rotation of the first electrodes 29 with respect to the second electrodes 30 and torsion of the arms 22, 26 about the respective axes A, B. In this way, controlled rotation of the platform 21 (and of the reflecting surface, not shown) with respect to axes A, B is obtained and thus scanning in the horizontal and in the vertical directions.
In the above applications, in particular for scene or gesture recognition, high positioning precision of the mirror element 5 is required, i.e., accurate knowledge of the position thereof.
Consequently, position acquisition systems are under study tailored for MEMS devices and based upon position sensors integrated in the mirror element 5.
For instance, United States Patent Application Publication No. 2011/0199284 (incorporated by reference) describes a piezoresistive sensor formed by at least one bridge element arranged near the first torsion springs (referred to as “flexures”), between the platform and the frame so that the torsional component of the output signal is amplified and the non-torsional component is attenuated or even eliminated.
The above known solution enables elimination of the components of the output signal due to undesired effects (disturbance), for example, components due to process spread and variations of material characteristics, which cause deformations of the torsion springs other than deformation controlled by the actuation system, in particular torsion spring bending.
It does not, however, enable discrimination of deformations of the structure that involve a spurious torsion of the torsion springs, for example, when (in the embodiment of FIG. 4) actuation of the second arms 26 causes a torsional deformation of the suspended frame 23 and of the corresponding torsion springs 22. Furthermore, with the sensors described in the above patent publication, it is not possible to control both the angular positions and it is necessary to have respective sensors.
There is a need in the art to provide a position detecting system that overcomes the limitations of prior art solutions.